Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session T18: Focus Session: Biological-Synthetic Hybrid Materials III |
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Sponsoring Units: DPOLY DBP Chair: Francis Starr, Wesleyan University Room: B117 |
Wednesday, March 17, 2010 2:30PM - 2:42PM |
T18.00001: Self-assembling Phage-Quantum Dot Nanocomplexes for Quantitative Biodetection Matthew Clarke, HyeongGon Kang, Jeeseong Hwang Colloidal quantum dots (QDs) have been used for many biodetection applications because of their brightness and broad spectral coverage in multiplexed approaches. QD surfaces can be functionalized for bio-conjugation to enable self-assembly with other nanomaterials and biomolecules using biological or bio-inspired processes. We demonstrate a model bacterial detection system using phage-QD nanocomplexes. To engineer the nanocomplexes, we genetically modified phage to express lysine residues on the capsid region, resulting in biotin labeling during replication inside the host cell. The biotinylated phages were conjugated with QDs and employed for detection. Bacteriophages have specificity to bacteria, enabling targeted detection of specific strains. Brightness of QDs enables high-throughput optical detection. The properties of nanocomplexes and detection limit/sensitivity were quantitatively evaluated using integrated differential interference contrast and fluorescence microscopy and automated image-based cytometry technique. [Preview Abstract] |
Wednesday, March 17, 2010 2:42PM - 2:54PM |
T18.00002: Single mode excitation of microtubules using a double slit ultrasound device Abdorreza Samarbakhsh, Jack Tuszynski Microtubules (MTs) are a major part of the cytoskeleton of all eukaryotic cells. They directly contribute to the process of cell division by forming mitotic spindles and providing force for the segregation of chromosomes. In this work we present analytical solutions to the problem of the vibrational dynamics of a MT that is attached at its two ends (of relevance mitosis) inside a viscous solution, driven by an ultrasound plane wave. We have shown that the ultrasound plane waves excite all modes of microtubule vibration at the same time which prevents the generation of resonance with a large enough amplitude. Specifically, when the MT is excited with a plane wave, the amplitude of each mode is inversely proportional to its mode number. Having a large enough amplitude for the vibrational effect is crucial in order to maximize the bending moment of a MT. Also achieving resonance is important in order to establish frequency control on the system. In order to overcome this difficulty, we propose to excite just a single mode of the MT using an ultrasound generation device using a double slit design that allows for both the frequency control and optimized energy transfer to the MT. [Preview Abstract] |
Wednesday, March 17, 2010 2:54PM - 3:06PM |
T18.00003: Design, Characterization and Application of Biocompatible Micro- and Nano-Particles for Dynamic X-ray Visual Imaging Sungsook Ahn, Sung Yong Jung, Sang Joon Lee For a dynamic X-ray visual imaging, contrast agents in particle forms are developed in micro- and nano-scales. First, water- soluble contrast agents are encapsulated into biocompatible polymeric microparticles. The fabricated polymeric microparticles are controlled in terms of the size, degree of crosslinking and encapsulation efficiency of the X-ray contrast agent. Second, gold is designed into functionalized nanoparticles. Properly controlled aggregates or encapsulated into human red blood cell (RBC), the surface modified gold nanoparticles are used as excellent X-ray contrast agent. The behaviors of the particles in the solution state are described by hydrodynamic size (DH) and zeta-potential (?-potential). The microscopic structures of the gold nanoparticle aggregates are determined by scanning electron microscopy (SEM) and zone-plate X-ray microscopy. The designed particulate biocompatible tracers are successfully used as excellent X-ray imaging tracers which enable the dynamic study in various biological fluid flows. [Preview Abstract] |
Wednesday, March 17, 2010 3:06PM - 3:18PM |
T18.00004: Ultrafast Polarization Phase Selective (PPS) molecular radiation signatures helping molecules-build- molecules that can do what no molecule has done before Kresimir Rupnik, Judy Cherian, Takahisa Tokumoto, Stephen McGill Molecular level process-control in enzyme-like hybrid bio-systems and related molecular electronics, redox or energy conversion and storage devices requires understanding of fast electronic, nuclear and photonic reagents at many different sites. Indeed, the integration of various components into a single molecular architecture often requires considerable knowledge of structural details not readily available. Our studies based on the combination of carefully tailored ultrafast PPS THz, optical, and soft X pulses, low and high magnetic fields and temperatures provide us with highly selective signatures of electronic and nuclear modes involved in such site-specific molecular physics and chemistry. Examples of recently identified coherent modes of significant importance in molecules-building-molecules bottom --up construction of larger integrated systems are presented and discussed. [Preview Abstract] |
Wednesday, March 17, 2010 3:18PM - 3:30PM |
T18.00005: Mesoscopic Simulations of the Insertion of the Amphiphilic Nanotubes into Lipid Bilayers Meenakshi Dutt, Michael Nayhouse, Olga Kuksenok, Alexander Alexeev, Steven R. Little, Anna C. Balazs Using Dissipative Particle Dynamics (DPD) simulations, we investigate the interactions between amphiphilic nanotubes and a lipid bilayer membrane. Each nanotube encompasses an ABA triblock (TB) architecture, with a hydrophobic stalk and two hydrophilic ends. To allow controlled transport through the nanotube, we introduce hydrophilic tethers at one or both ends of the nanotube. Individual lipids are composed of a hydrophilic head group and two hydrophobic tails. We begin with a stable lipid bilayer membrane immersed in a hydrophilic solvent, and introduce the nanotube into the surrounding solution. The energetically unfavorable interaction between the solvent and the hydrophobic segment of the nanotube could potentially drive them to penetrate the membrane, with the hydrophobic stalk being buried within the hydrophobic domains of the bilayer. This process, however, depends upon the hydrophobic fraction of the nanotube, the degree of hydrophobic mismatch between the nanotube and the bilayer, and the presence of hydrophilic end-tethers. We isolate the conditions that promote the insertion of the synthetic nanotubes into membrane. The simulations are supported by free energy calculations for the amphiphilic nanotube-lipid-solvent system. Ultimately, these embedded synthetic nanotubes could be used to regulate the passage of molecules through synthetic membranes. [Preview Abstract] |
Wednesday, March 17, 2010 3:30PM - 3:42PM |
T18.00006: Attachment dynamics of Photosystem I on nano-tailored surfaces for photovoltaic applications Dibyendu Mukherjee, Barry D. Bruce, Bamin Khomami Photosystem I (PSI), a biological photodiode, is a supra-molecular protein complex that charge separates upon exposure to light. Effective use of photo-electrochemical activities of PSI for hybrid photovoltaic (PV) device fabrications requires optimal encapsulation of these proteins onto organic/ inorganic substrates. Our results indicate that various experimental parameters alter the surface attachment dynamics of PSI deposited from colloidal aqueous buffer suspensions onto OH-terminated alkanethiolate/Au SAM substrates, thereby resulting in complex structural arrangements which affect the electron transfer and capture pathway of PSI. We present surface topographical, specific adsorption and polarization fluorescence characterizations of PSI/Au SAM substrates to elucidate the protein-surface interaction kinetics as well as the directional attachment dynamics of PSI. Our final goal is to enable site-specific homogeneous attachment of directionally aligned PSI onto chemically tailored nano-patterned substrates. [Preview Abstract] |
Wednesday, March 17, 2010 3:42PM - 3:54PM |
T18.00007: Branched Actin Networks and Optimized Orientation D.A. Quint, J.M. Schwarz Experiments measuring the orientation of extracted, \textit{in vivo}-grown branched actin filaments with respect to the leading edge show a distribution prominently peaked at $\pm35^{\circ}$, which is half of the measured branching angle. To understand this result, we model the successive generations of polymerizing, branched actin filaments as a set of coupled kinetic equations with a branching (birth) rate that depends on the orientation of the filament with respect to the leading edge of the cell and a constant capping (death) rate. We find in steady state that the orientation angle of the filament with respect to the leading edge optimizing for survival is in agreement with the observed values of $\pm35^{\circ}$. A previous rate equation based model, introduced by Maly and Borisy, yields the same result but with an orientation dependent capping (death) rate. Given these similar outcomes, we investigate whether this result is generic for models where the birth and death rates are dependent on more general functions of the filament orientation. We also study the effects of fluctuations in the branching angle on the optimal orientation angle. [Preview Abstract] |
Wednesday, March 17, 2010 3:54PM - 4:06PM |
T18.00008: Quantitative Analysis of Statics and Dynamics of Actin Cables in Fission Yeast Eddy Yusuf, Jian-Qiu Wu, Dimitrios Vavylonis The assembly of actin and tubulin proteins into long filaments and bundles, i.e. closely-packed filaments, underlies important cellular processes such as cell motility, intracellular transport, and cell division. Recent theoretical and experimental work has addressed the nonequilibrium dynamics of single microtubules within live cells [1]. Actin filaments usually form dense networks that prevents microscopic imaging of individual filaments or bundles. Here, we studied actin dynamics using fission yeast that has low-density actin cytoskeleton consisting of actin cables (actin bundles aligned along the long axis of the cell) and ``actin patches.'' Yeast cells expressing GFP-CHD were imaged by 3D confocal microscopy. Stretching open active contours [2] were used to segment and track individual actin cables. We analyzed their curvature distribution, the tangent correlation, and the temporal bending amplitude fluctuations. We contrast our findings to equilibrium fluctuating semiflexible polymers and to microtubules in cells. We calculate the important time and length scales for the actin cables. We also discuss our findings within the broad context of understanding actin assembly in cells. [1] C. P. Brangwynne et. al., Phys. Rev. Lett. 100, 118104 (2008) [2] H. Li et. al., Proc. of the IEEE Int'l Symposium on Biomedical Imaging: From Nano to Macro, ISBI'09 [Preview Abstract] |
Wednesday, March 17, 2010 4:06PM - 4:18PM |
T18.00009: Actin Polymerization Driven Mitochondrial Transport in Mating S. cerevisiae by Fourier Imaging Correlation Spectroscopy Eric Senning, Andrew Marcus The dynamic microenvironment of cells depends on macromolecular architecture, equilibrium fluctuations, and non-equilibrium forces generated by cytoskeletal proteins. We studied the influence of these factors on the motions of mitochondria in mating \textit{S. cerevisiae }using Fourier imaging correlation spectroscopy (FICS). Our measurements provide detailed, length scale dependent information about the dynamic behavior of mitochondria. We investigate the influence of the actin cytoskeleton on mitochondrial motion, and make comparisons between conditions in which actin network assembly and disassembly is varied, either by using disruptive pharmacological agents, or mutations that alter the rates of actin polymerization. We find that non-equilibrium forces associated with actin polymerization lead to a 1.5-fold enhancement of the long-time mitochondrial diffusion coefficient, and a transient sub-diffusive temporal scaling of the mean-square displacement. Our results lend support to an existing model in which these forces are directly coupled to mitochondrial membrane surfaces. [Preview Abstract] |
Wednesday, March 17, 2010 4:18PM - 4:30PM |
T18.00010: Kinetic Proofreading of Cytoskeletal Structures Douglas Swanson, Ned Wingreen Cytoskeletal polymer dynamics play a role in cellular processes as varied as reproduction, locomotion, and intracellular transport. Microtubules are cytoskeletal biopolymers that grow by accumulating tubulin subunits bound to guanosine triphosphate (GTP). The subunits hydrolyze GTP to guanosine diphosphate (GDP), causing a conformational change in the protein that destabilizes the microtubule. GDP-bound subunits tend to depolymerize, leading to stochastic microtubule disassembly in a process known as dynamic instability. Over time polymerization and depolymerization come to steady state, leading to a local steady-state concentration of tubulin subunits. This may be viewed as a kind of ``kinetic proofreading,'' in which the system consumes energy actively to ``proofread'' the steady-state subunit concentration. We suggest that the same mechanism could also ``proofread'' between different cytoskeletal structures. For example, we show that a small free-energy difference between two polymer orientations, combined with dynamic instability, can strongly drive the system towards the lower free-energy state. This might help to explain the long-time stability of many cytoskeletal structures despite the short-time rapid turnover of the individual subunits. [Preview Abstract] |
Wednesday, March 17, 2010 4:30PM - 4:42PM |
T18.00011: Poisson's ratio and microrheology of composite elastic materials with rigid rods. Moumita Das, F.C. MacKintosh We study both the macroscopic and micro-mechanical response of composites of rods embedded in elastic media. We calculate the response functions, and bulk elastic constants as functions of rod density. We find two fixed points for Poisson's ratio in 3D composites: there is an unstable fixed point for Poisson's ratio=1/2 (an incompressible system) and a stable fixed point for Poisson's ratio=1/4 (a compressible system). This stable fixed point is found to be robust with respect to rod density and extensibility. At high concentration, we recover the results for affine rod networks. These results may help to explain recent experiments [Physical Review Letters 102, 188303 (2009)] that reported compressibility for composites of microtubules and F-actin networks. [Preview Abstract] |
Wednesday, March 17, 2010 4:42PM - 4:54PM |
T18.00012: First principles study of biomineral hydroxyapatite Alexander Slepko Hydroxyapatite (HA) [Ca$_{10}$(PO$_{4})_{6}$(OH)$_{2}$] is one of the most abundant materials in mammal bone. It crystallizes within the spaces between the tropocollagen chains and strengthens the bone tissue. The mineral content of human bone increases with age reaching a maximum value from which it starts to decrease leading to diseases such as osteomalacia. Therefore, an emergent application of this study is bone repair and the production of synthetic bone. Despite its importance, little is known about the growth of HA crystallites in bones. Nor is it well understood how the HA attaches to protein chains and interacts with the surrounding aqueous solution. Using density functional theory (DFT) we calculate the theoretical ground state structure, electronic and vibration properties of hexagonal HA. We find several low energy structures and analyze the energy barriers for spontaneous phase transitions. We calculate the phonon density of states and study the surface energetics for different orientations. We identify the surfaces with highest reactivity using the frontier orbital approach and analyze interactions between these surfaces and water molecules/amino acids. [Preview Abstract] |
Wednesday, March 17, 2010 4:54PM - 5:06PM |
T18.00013: Experimental studies of the direct flexoelectric effect in bone materials John Fu The piezoelectric effect in biological tissues has been attracting research interest due to the hypothesis that it may behave as a biological transducer, which can convert external stimuli into biologically-recognizable signals capable of controlling growth or resorptive processes. The piezoelectric effect in dried bone materials was first observed in 1957 [1]. A link between the effect and the adaptive response of bone cells was proposed in 1970 [2]. In this paper, we report our recent measurements on the direct flexoelectric effect in bone materials. Our specimens are both dried and wet bones. The origin of both piezoelectricity and flexoelectricity in bone may be ascribed to the crystalline alignment of the micelle of collagen molecules. The Curie group symmetries of the configuration of collagen fibres in the bone texture demonstrate the existence of both effects. However, our experimental results show that the piezoelectric responses in bone materials may be dominated by flexoelectricity at the micro and nano scales. Finally, we propose a link between the flexoelectric effect and bone spur (osteophyte). [1] E. Fukada and I. Yasuda, J. Phys. Soc. Jpn. 12, 1158 (1957). [2] A. Marino and R. Becker, Nature 228, 78 (1970). [Preview Abstract] |
Wednesday, March 17, 2010 5:06PM - 5:18PM |
T18.00014: Morphology changes caused by seawater ions in templated calcite crystals. Benjamin Stripe, Ahmet Uysal, Pulak Dutta It has been shown that the presence of Mg in ratios approximately equal to those found in seawater, during the organic-monolayer-templated nucleation of CaCO$_{3}$, significantly changes the morphology of the nucleating crystals. Crystals nucleated from supersaturated subphases of CaCO$_{3}$ under floating arachidic sulfate monolayers grow as (001) oriented tetragonal pyramids [1]. We have found that crystals nucleating under arachidic sulfate from supersaturated solutions of CaCO$_{3}$ containing approximately 5:1 Mg:CaCO$_{3}$ grow as (001) hexagonal prisms, which express (100) or (110) faces. The crystal-water surface energy of these faces is almost twice that of the (104) face expressed on the tetragonal pyramids [2]. The growth of two morphologies displaying different surfaces but the same (001) orientation suggests that epitaxy, when present, plays a larger role than surface energetics alone. Our studies of Mg-concentration-dependent changes in morphology will also be presented. \newline \newline [1] Kewalramani, S. et. al.'', Langmuir, 24, 10579, 2008 \newline [2] Duffy, D.and Harding, J, Langmuir, 20, 7630, 2004 [Preview Abstract] |
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